Influenza A(H3N2), the predominant type of influenza virus
isolated
in the United States during the 1987-88 season, exhibited antigenic
drift from previous epidemic strains (1). Many of the isolates
resembled two strains first recognized in China during 1987,
A/Sichuan/2/87 and A/Shanghai/11/87. Outbreaks reported during
1987-88
in the United States that were associated with these viruses
occurred
in all age groups, including residents of nursing homes. Antigenic
variants of influenza B also circulated during the 1987-88 season,
with
most isolates resembling B/Victoria/2/87 (2). The number of
influenza B
virus isolates increased late in the season when the first
outbreaks
associated with this virus were reported; at the same time,
influenza
A(H3N2) declined. Influenza A(H1N1) viruses similar to
A/Taiwan/1/86,
the predominant influenza virus during the 1986-87 season (3), were
the
least frequently isolated viruses during the 1987-88 season and
were
associated with only one possible outbreak, which occurred among
college students. The number of influenza A(H1N1) virus isolates
also
increased late in the season.

Sources for surveillance of influenza were the same as for the
1986-87 season (3) with these exceptions:

Sentinel physician surveillance network. The number of
reporting
physicians increased to 141. A subgroup of 40 physicians collected
nasopharyngeal specimens from selected cases and immediately
submitted
those specimens for virus processing. Rapid culture confirmation
techniques were used to identify and report positive results to the
physicians within 24 hours of test results or 5-6 days of specimen
collection. The culture confirmation technique identified the type
of
influenza virus but not the subtype of influenza A.

World Health Organization (WHO) collaborating laboratories.
Fifty-three (instead of the previous 64) laboratories, based in
state
or local health departments, universities, or hospitals, reported
by
postcard the number of specimens tested and the number and type of
influenza viruses isolated for each week from early October through
mid-May. Data from the other WHO collaborating laboratories were
reported through the Epidemiologic Surveillance Project (4).

Epidemiologic Surveillance Project (ESP). In this project, case
reports of culture- confirmed influenza were submitted
electronically
to CDC from state health departments in Georgia, Kentucky, North
Carolina, South Carolina, Texas, and Vermont. All cases identified
by
WHO collaborating laboratories and other participating laboratories
in
these states were reported. Information reported for each case
included
patient age, county of residence, date of specimen collection, date
of
report to state, type of influenza virus identified, and, if known,
the
subtype for type A influenza viruses.

The first suspected outbreak of influenza A(H3N2) occurred in
cruise ship passengers who were touring Alaska during August (5).
In
October, a probable outbreak of influenza A(H3N2) occurred among
American tourists traveling in the Orient aboard a cruise ship (6).
The
first reported domestic outbreak of influenza-like illness occurred
in
November in preschool children in Colorado; influenza A(H3N2) was
isolated from a specimen obtained from the index patient (7).
Sporadic
isolates of influenza B were also reported early in the season from
Arizona, Hawaii, and Wisconsin (8). However, the first reported
outbreak of influenza B occurred in February in a Connecticut
nursing
home. Most reported outbreaks of influenza A(H3N2) and influenza B
occurred in nursing homes or other long-term-care settings.

According to reports by sentinel physicians, the mean
percentage of
total weekly patient visits associated with influenza-like illness
was
4.8% (Figure 1). Sentinel physicians also reported each week
whether an
outbreak of influenza is occurring among their patients. Outbreaks
were
reported primarily during January and February by physicians in the
western and central regions of the country and during February and
March by physicians from the eastern regions.

Morbidity reports from state epidemiologists indicated that
peak
influenza activity occurred during February and early March (Figure
1).
Widespread or regional outbreaks were reported in 44 states and the
District of Columbia (Figure 2). Outbreaks in the western and
central
regions of the country were reported earlier than those in the
eastern
regions.

WHO collaborating laboratories tested 26,732 specimens for
influenza viruses. Isolates were recovered from 2,532 (9.5%) of
these
specimens. Nineteen hundred (75%) of the isolates were influenza
A(H3N2), 430 (17.0%) were influenza B, and 202 (8.0%) were
influenza
A(H1N1) (Figure 3). Isolation of influenza A(H3N2) peaked during
February, while influenza B and influenza A(H1N1) peaked during
late
March (Figure 1). Sentinel physicians submitted an additional 420
specimens for testing; 119 (28.3%) of these were positive for
influenza
viruses. Of the positive specimens, 110 (92.4%) were type A, and
nine
(7.6%) were type B influenza.

Combining all laboratory reports, influenza A(H3N2) viruses
were
reported from 49 states and the District of Columbia; influenza B,
from
26 states in all regions of the country and the District of
Columbia;
and influenza A(H1N1), from 19 states primarily in the eastern,
central, and southern regions of the country.

The proportion of deaths associated with pneumonia and
influenza
(P&I) reported from 121 cities exceeded the epidemic threshold for
9
weeks, from the week ending February 20 through the week ending
April
16 (Figure 4). Eighty-six percent of the P&I deaths reported
occurred
in persons greater than or equal to 65 years of age. The 1987-88
season
was the fifth year in the last decade that influenza A(H3N2)
predominated. In each of the 5 years, excess mortality associated
with
P&I has occurred.

Preliminary analysis of the data received through ESP indicates
the
relative proportions of influenza virus types reported through this
system were similar to those reported on postcards by the other WHO
collaborating laboratories. Of the 661 isolates reported through
ESP,
508 (76.9%) were type A(H3N2), 94 (14.2%) were type B, 20 (3.0%)
were
type A(H1N1), and 39 (5.9%) were type A viruses, not subtyped. Of
the
ESP isolate reports, 354 (53.6%) were reported from Harris County,
Texas, where special influenza studies are conducted by the
Influenza
Research Center at the Baylor College of Medicine. The mean age of
patients from whom isolates were recovered was 27 years for
influenza A(H3N2), 20 years for influenza A(H1N1), and 19 years for
influenza B. The median number of days between specimen collection
and
the date the results of virus testing were reported to the state
epidemiologist was 27 days. Most reports were then transmitted to
CDC
within 1 week.
Reported by: Participating state and territorial epidemiologists
and
state laboratory directors. WHO Collaborating Laboratories.
Sentinel
Physicians of the American Academy of Family Physicians.
Participating
Veterans Administration Hospitals. Letterman Army Medical Center,
San
Francisco, California. Hackensack Hospital, Hackensack, New Jersey.
Strong Memorial Hospital, Rochester, New York. Vanderbilt Univ,
Nashville, Tennessee. Influenza Research Center, Baylor College of
Medicine, Houston; 5th Army Medical Laboratory, Fort Sam Houston;
USAF
School of Aerospace Medicine, Epidemiology Div. Brooks AFB, Texas.
Div
of Surveillance and Epidemiologic Studies, Epidemiology Program
Office;
WHO Collaborating Center for Influenza, Influenza Br, Div of Viral
Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: During the 1986-87 season, influenza A(H1N1) was
the
most frequently isolated influenza virus. Since its reappearance in
1977, A(H1N1) has primarily been associated with morbidity in
younger
persons. In contrast, influenza A(H3N2)--the predominant strain
during
the 1987-88 season (3)-- causes morbidity in all age groups and
mortality in the elderly. In 1986-87, only 2.3% of all influenza
isolates were from persons greater than or equal to 65 years of
age,
while in 1987-88, 20.7% of the influenza A(H3N2) isolates reported
by
WHO collaborating laboratories reporting through the postcard
system
were from persons in this age group (Table 1) (3). The excess
mortality
associated with P&I is consistent with an increased occurrence of
influenza in the elderly (Figure 4).

The 1987-88 influenza epidemic was associated with strains that
exhibited antigenic drift from the strain that had been included in
the
vaccine. However, because these variations were not recognized
until
the fall of 1987, the trivalent influenza vaccine could not be
modified
to include the new variant. As a result, the efficacy of the
vaccine,
at least in certain high-risk persons, may have been reduced.

Efforts to improve influenza control are emphasizing rapid
detection and reporting of influenza viruses--including those
circulating in the Far East--in time to consider incorporating
these
viruses into the influenza vaccine. In addition, surveillance in
the
United States augmented by laboratory support enhances the
monitoring
of influenza, often before outbreaks occur, and can contribute to
influenza control by enabling the use of antiviral agents in
locations
where influenza A is circulating.

The ESP and Sentinel Physician Surveillance have expanded
options
for epidemiologic surveillance of influenza. The ESP for influenza
surveillance was first operated during the 1987-88 influenza season
and
provided data not reported by the postcard system. Specimen
collection
dates and additional case-specific information permit more detailed
epidemiologic analysis than the postcard reporting system, thereby
enhancing surveillance of both morbidity and viral isolation. The
results of the Sentinel Physician Surveillance Network, a pilot
study
in progress for several years, have demonstrated the feasibility of
a
relatively inexpensive method for rapid confirmation of influenza
in
specimens collected by family physicians and have provided prompt
feedback to these physicians (9).

References

CDC. Influenza--United States. MMWR 1988;37:207-9.

CDC. Update on influenza activity--United States and worldwide,
with recommendations for influenza vaccine composition for the
1988-89
season. MMWR 1988;37:241-4.

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